linux-kernel - Re: [PATCH v8 1/3] iommu/arm-smmu: add NVIDIA implementation for dual ARM MMU-500 usage

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Date:   Tue, 30 Jun 2020 20:03:04 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Krishna Reddy <vdumpa@...dia.com>
CC:     "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        Thierry Reding <treding@...dia.com>,
        "Yu-Huan Hsu" <YHsu@...dia.com>, Sachin Nikam <Snikam@...dia.com>,
        Pritesh Raithatha <praithatha@...dia.com>,
        Timo Alho <talho@...dia.com>,
        Bitan Biswas <bbiswas@...dia.com>,
        Mikko Perttunen <mperttunen@...dia.com>,
        Nicolin Chen <nicolinc@...dia.com>,
        Bryan Huntsman <bhuntsman@...dia.com>,
        "nicoleotsuka@...il.com" <nicoleotsuka@...il.com>
Subject: Re: [PATCH v8 1/3] iommu/arm-smmu: add NVIDIA implementation for dual
 ARM MMU-500 usage


On 30/06/2020 18:16, Krishna Reddy wrote:
>> OK, well I see what you are saying, but if we intended to support all 3 for Tegra194, then we should ensure all 3 are initialised correctly.
> 
> The driver intend to support up to 3 instances. It doesn't really mandate that all three instances be present in same DT node.
> Each mmio aperture in "reg" property is an instance here. reg = <inst0_base, size>, <inst1_base, size>, <inst2_base, size>;
> The reg can have all three or less and driver just configures based on reg and it works fine.

So it sounds like we need at least 2 SMMUs (for non-iso and iso) but we
have up to 3 (for Tegra194). So the question is do we have a use-case
where we only use 2 and not 3? If not, then it still seems that we
should require that all 3 are present.

The other problem I see here is that currently the arm-smmu binding
defines the 'reg' with a 'maxItems' of 1, whereas we have 3. I believe
that this will get caught by the 'dt_binding_check' when we try to
populate the binding.

>> It would be better to query the number of SMMUs populated in device-tree and then ensure that all are initialised correctly.
> 
> Getting the IORESOURCE_MEM is the way to count the instances driver need to support.  
> In a way, It is already querying through IORESOURCE_MEM here. 

Yes I was wondering that. I think we just need to decide if the 3rd SMMU
is optional or not. The DT binding should detail and min and max supported.	

Jon

-- 
nvpublic